A 10 metre (33-ft) humpback whale spotted in the River Thames earlier this week has been found dead just east of London and will now be handed over to scientists for an autopsy.

The whale was found dead in shallow water at Greenhithe in Kent on Tuesday and towed late at night from below the QE2 Bridge to a Port of London Authority facility at Gravesend.

‘It is incredibly sad - I was literally two feet from this dead whale,’ said Martin Garside, a spokesman for the Port of London authority who helped tow the whale.

‘It was both poignant and a bit eerie really - road traffic was thundering overhead on the busiest motorway in Britain and oblivious to all the people in the cars and lorries there was a 10-metre long beautiful whale floating dead beneath them.’

The whale was so large that the patrol boats struggled to tow it.

‘It was bigger than the patrol boat I was in, no doubt about it, and it was getting on for the length of the bigger patrol boat so I would say minimum 10 metres,’ Garside said.

The Zoological Society of London has a Cetacean Strandings Investigation Programme which will investigate why the whale entered the area and why it died.

‘At this stage it's too early to do more than speculate on why this may have happened,’ the programme manager, Rob Deaville, said in a statement.

Two other humpback whales have previously been recorded in the Thames and its wider estuary region, one in 2009 and one in 2013. Both of those animals died.

The humpback whale found at Greenhithe on Tuesday was the fifth of its species to be recorded stranded on the UK coast so far this year, compared with an annual average of one to two.

Deaville said it was hard to generalise about stranding numbers because different species faced different threats, and strandings could occur for a wide variety of reasons.

Globally, humpback whale populations are increasing in size after a moratorium on commercial whaling was put into effect in 1986. More strandings are occurring as the population increases, but there are also cases of mortality related to human activities such as shipping and fishing.